netstat) than they saw using older versions. When the server closes a TCP connection, it sends a packet with the FIN bit sent to the client, which then responds with a packet with the ACK bit set. The client then sends a packet with the FIN bit set to the server, which responds with an ACK and the connection is closed. The state that the connection is in during the period between when the server gets the ACK from the client and the server gets the FIN from the client is known as FIN_WAIT_2. See the TCP RFC for the technical details of the state transitions.
The FIN_WAIT_2 state is somewhat unusual in that there is no timeout defined in the standard for it. This means that on many operating systems, a connection in the FIN_WAIT_2 state will stay around until the system is rebooted. If the system does not have a timeout and too many FIN_WAIT_2 connections build up, it can fill up the space allocated for storing information about the connections and crash the kernel. The connections in FIN_WAIT_2 do not tie up an httpd process.
If you are lucky, this means that the buggy client will fully close the connection and release the resources on your server. However, there are some cases where the socket is never fully closed, such as a dialup client disconnecting from their provider before closing the client. In addition, a client might sit idle for days without making another connection, and thus may hold its end of the socket open for days even though it has no further use for it. This is a bug in the browser or in its operating system's TCP implementation.
The clients on which this problem has been verified to exist:
This does not appear to be a problem on:
It is expected that many other clients have the same problem. What a client should do is periodically check its open socket(s) to see if they have been closed by the server, and close their side of the connection if the server has closed. This check need only occur once every few seconds, and may even be detected by a OS signal on some systems (e.g., Win95 and NT clients have this capability, but they seem to be ignoring it).
Apache cannot avoid these FIN_WAIT_2 states unless it disables persistent connections for the buggy clients, just like we recommend doing for Navigator 2.x clients due to other bugs. However, non-persistent connections increase the total number of connections needed per client and slow retrieval of an image-laden web page. Since non-persistent connections have their own resource consumptions and a short waiting period after each closure, a busy server may need persistence in order to best serve its clients.
As far as we know, the client-caused FIN_WAIT_2 problem is present for all servers that support persistent connections, including Apache 1.1.x and 1.2.
One possible (and most likely) source for additional FIN_WAIT_2 states
is a function called
lingering_close() which was added
between 1.1 and 1.2. This function is necessary for the proper
handling of persistent connections and any request which includes
content in the message body (e.g., PUTs and POSTs).
What it does is read any data sent by the client for
a certain time after the server closes the connection. The exact
reasons for doing this are somewhat complicated, but involve what
happens if the client is making a request at the same time the
server sends a response and closes the connection. Without lingering,
the client might be forced to reset its TCP input buffer before it
has a chance to read the server's response, and thus understand why
the connection has closed.
See the appendix for more details.
We have not yet tracked down the exact reason why
lingering_close() causes problems. Its code has been
thoroughly reviewed and extensively updated in 1.2b6. It is possible
that there is some problem in the BSD TCP stack which is causing the
observed problems. It is also possible that we fixed it in 1.2b6.
Unfortunately, we have not been able to replicate the problem on our
tcp_fin_wait_2_flush_interval, but the default should be appropriate for most servers and improper tuning can have negative impacts.
SO_LINGERsocket option which is enabled by Apache. This parameter can be adjusted by using
nettuneto modify parameters such as
tcp_keepstop. In later revisions, there is an explicit timer for connections in FIN_WAIT_2 that can be modified; contact HP support for details.
The following systems are known to not have a timeout:
There is a patch available for adding a timeout to the FIN_WAIT_2 state; it was originally intended for BSD/OS, but should be adaptable to most systems using BSD networking code. You need kernel source code to be able to use it. If you do adapt it to work for any other systems, please drop me a note at email@example.com.
lingering_close()function. This will result in that section of code being similar to that which was in 1.1. If you do this, be aware that it can cause problems with PUTs, POSTs and persistent connections, especially if the client uses pipelining. That said, it is no worse than on 1.1, and we understand that keeping your server running is quite important.
To compile without the
lingering_close() function, add
-DNO_LINGCLOSE to the end of the
EXTRA_CFLAGS line in your
Configure and rebuild the server.
SO_LINGERas an alternative to
SO_LINGERthat can be set with
setsockopt(2). It does something very similar to
lingering_close(), except that it is broken on many systems so that it causes far more problems than
lingering_close. On some systems, it could possibly work better so it may be worth a try if you have no other alternatives.
To try it, add
-DUSE_SO_LINGER -DNO_LINGCLOSE to the end of the
EXTRA_CFLAGS line in your
Configure and rebuild the server.
NOTE: Attempting to use
lingering_close() at the same time is very likely to do
very bad things, so don't.
The exact way to increase them may depend on your OS; look
for some reference to the number of "mbufs" or "mbuf clusters". On
many systems, this can be done by adding the line
n is the number of
mbuf clusters you want to your kernel config file and rebuilding your
Below is a message from Roy Fielding, one of the authors of HTTP/1.1.
If a server closes the input side of the connection while the client is sending data (or is planning to send data), then the server's TCP stack will signal an RST (reset) back to the client. Upon receipt of the RST, the client will flush its own incoming TCP buffer back to the un-ACKed packet indicated by the RST packet argument. If the server has sent a message, usually an error response, to the client just before the close, and the client receives the RST packet before its application code has read the error message from its incoming TCP buffer and before the server has received the ACK sent by the client upon receipt of that buffer, then the RST will flush the error message before the client application has a chance to see it. The result is that the client is left thinking that the connection failed for no apparent reason.
There are two conditions under which this is likely to occur:
The solution in all cases is to send the response, close only the write half of the connection (what shutdown is supposed to do), and continue reading on the socket until it is either closed by the client (signifying it has finally read the response) or a timeout occurs. That is what the kernel is supposed to do if SO_LINGER is set. Unfortunately, SO_LINGER has no effect on some systems; on some other systems, it does not have its own timeout and thus the TCP memory segments just pile-up until the next reboot (planned or not).
Please note that simply removing the linger code will not solve the problem -- it only moves it to a different and much harder one to detect.